1. Field
The present invention relates generally to data communications, and more specifically to techniques for optimizing channel estimation in an orthogonal frequency division multiplexing (OFDM) communication system.
2. Background
OFDM is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (N) orthogonal subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. In an OFDM communication system, each subband is associated with a respective subcarrier that may be modulated with data.
In a wireless communication system, a radio frequency (RF) modulated signal may travel via a number of signal paths from a transmitter to a receiver. If the signal paths have different delays, then the signal arriving at the receiver would include multiple instances of the transmitted signal with different gains and delays. This time dispersion in the wireless channel causes frequency selective fading, which is characterized by a frequency response that varies across the system bandwidth. In an OFDM system, the N subbands may thus experience different effective channels and may consequently be associated with different complex channel gains.
An accurate estimate of the wireless channel between the transmitter and the receiver is normally needed in order to effectively receive data on the available subbands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of modulation symbols that are known a priori by the receiver, the channel response can be estimated as the ratio of the received pilot symbol over the transmitted pilot symbol for each subband used for pilot transmission.
In order to obtain a reliable channel estimate and extract data from a particular channel, channel signal energy must be reliably distinguished from interference energy in the channel estimate by a process referred to as “thresholding”. A thresholding process uses a threshold to determine whether a given channel element, or tap, has sufficient channel energy to be retained, or has insufficient energy and should be discarded, or zeroed out. If the energy in a channel is above the threshold, it is retained as channel energy. If the energy is below the threshold it may be discarded as.
Thresholds are known to be computed based on various factors and in various manners. Traditionally, the threshold can be applied to the channel estimate after Automatic Gain Control (AGC). The AGC operation scales the received signal such that the sum of channel energy and interference energy is kept fixed. The threshold can then be a fixed value for each data mode and can be computed based on the total or average energy of the channel response estimate after AGC. The use of the fixed threshold ensures that (1) the thresholding is not dependent on variations in the received energy before AGC and (2) the channel taps that are present but with low signal energy are not zeroed out. An absolute, or fixed, threshold can be computed based on the signal to interference ratio (C/I) required for a given data mode, the lowest energy expected for the received pilot symbols, and so on. The use of the fixed threshold forces the tap to meet some minimum value in order to be retained. The threshold can also be computed based on a combination of factors. For example, the threshold can be computed based on the energy of the channel impulse response estimate and further constrained to be equal to or greater than a predetermined minimum value.
The use of fixed thresholding techniques in OFDM systems is disclosed in U.S. patent application Ser. No. 10/741,524, entitled “CHANNEL ESTIMATION FOR AN OFDM COMMUNICATION SYSTEM WITH INACTIVE SUBBANDS,” assigned to the assignee of the present invention and incorporated by reference herein. The use of fixed thresholding techniques in OFDM systems is further disclosed in U.S. patent application Ser. No. 11/366,779, entitled “CHANNEL ESTIMATE OPTIMIZATION FOR MULTIPLE TRANSMIT MODES”, assigned to the assignee of the present invention and incorporated by reference herein. Additional details on a method for obtaining relative WIC based C/I measurements for deriving set relative thresholds are described in U.S. patent application Ser. No. 11/516, 910 entitled “METHODS AND APPARATUS FOR NOISE ESTIMATION IN A COMMUNICATION SYSTEM”, assigned to the assignee of he present invention, and incorporated by reference herein. C/I measurements for deriving set relative thresholds may be further obtained during data symbol processing as described in U.S. patent application. Ser. No. 11/563,627 111, entitled “METHODS AND APPARATUS FOR SIGNAL AND INTERFERENCE ENERGY ESTIMATION IN A COMMUNICATION SYSTEM,” assigned to the assignee of the present invention, the disclosure of which is also incorporated by reference herein.
Traditionally determined thresholds may be suboptimum because they do not dynamically accommodate changing channel conditions. When thresholds are arbitrarily fixed by mode (data rate) and changing channel conditions become better than expected, channel taps below the traditional threshold may be discarded when they could be retained. Removing viable channel taps then creates interference in the channel. As new generations of OFDM communication systems are deployed, signal differences between wide and local area channels must also be accommodated for by thresholding techniques.
There is therefore a need in the art for an adaptive thresholding technique, which eliminates suboptimum threshold values by adjusting for varying channel conditions to eliminate interference where no channel energy is present, without discarding viable taps having useful channel energy. New thresholding techniques must also adaptively optimize threshold values for both wide and local area channels.